kpageflags: detect isolated KPF_THP folios

When folio is isolated, the PG_lru bit is cleared.  So the PG_lru check in
stable_page_flags() will miss this kind of isolated folios.  Use
folio_test_large_rmappable() instead to also include isolated folios.

Since pagecache supports large folios and the introduction of mTHP, the
semantics of KPF_THP have been expanded, now it indicates not only
PMD-sized THP.  Update related documentation to clearly state that KPF_THP
indicates multiple order THPs.

[[email protected]: directly use is_zero_folio(), per David]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ran Xiaokai <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Andrei Vagin <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Muhammad Usama Anjum <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Svetly Todorov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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